Plywood has been defined as a crossbonded assembly made of layers of veneer (a thin layer or sheet of wood) in combination with a core of lumber or of plies joined with an adhesive. The grain of alternate layers of veneer is crossed, generally at right angles, which provides a panel with improved strength and dimensional stability compared to the original wood.
To describe a veneer plywood completely it is necessary to indicate three criteria including the species or group of timber, the grade of veneer, usually confined to the face and back veneers, and the bond or type of adhesive used. These factors generally limit the application in which the plywood can be employed.
This invention will be concerned mainly with plywood having a novel type of adhesive, and with the adhesive which is used.
Casein, soy protein and various animal protein glues were the main plywood adhesives employed up until 1935, when the first phenolic resins were developed, primarily for exterior plywood use. Since 1935, most of the protein-based glues have been replaced by synthetic resins. These include urea-formaldehyde, melamine-formaldehyde, resorcinol-formaldehyde, phenol-formaldehyde, phenol/resorcinol-formaldehyde, polyvinyl alcohol and polyvinyl alcohol/urea-formaldehyde resins.
The most commonly used resins are the urea-formaldehyde and phenol-formaldehyde resins. Plywood adhesives made from urea-formaldehyde resins are used extensively for interior and intermediate grades of plywood. This covers the majority of hardwood plywood produced. Adhesives made from phenol-formaldehyde resins are used for softwood plywood production, and are also standard for exterior grades of plywood.
In most plywood adhesive formulations, the synthetic resin is combined with various fillers and extenders, which not only reduce the overall cost of the adhesive but also perform various functions in the adhesive system. Fillers are added primarily to reduce the cost of the plywood adhesive. Materials such as finely ground clays, wood-fiber based products, such as wood, bark, walnut shell and coconut shell flours, as well as fibrous residues from the production of furfural have been employed. In addition to cost reduction, these products add particulate matter to the adhesive. This particulate matter assists in reducing penetration of the resins into the wood and fills small cavities between veneers, thus preventing starved joints.
Extenders are usually amylaceous containing materials, such as cereal flours and starches. In phenol-formaldehyde glues, these extenders provide a source of starch, which becomes gelatinized under the highly alkaline conditions present in phenolic resin systems. The gelatinized starch provides the glue with increased viscosity and tack. These properties allow for more control of the glue during application, provide improved glue lines and limit penetration of the resin into the wood plies.
With urea-formaldehyde glues, most amylaceous materials are not satisfactory. Since urea-formaldehyde resins are neutral or slightly acidic and do not contain alkali, the starch does not become gelatinized to provide increased viscosity to the adhesive system. Precooked or pregelatinized starches have been considered as extenders in urea-formaldehyde glues. However, they are more expensive, more difficult to use, and are not commonly used in plywood manufacture today.
Wheat flours are the only principal starch-based extenders that are employed today in urea-formaldehyde plywood glues. These products are unique in that they can provide viscosity in certain adhesive systems without the starch portion of the flour becoming gelatinized. This property is usually associated with the gluten protein portion of wheat flours. Wheat gluten has considerably different properties compared to the protein portions of other cereals, such as corn, rice, sorghum, etc. The gluten protein of wheat is responsible for the dough forming properties which make wheat flours particularly useful in the production of bread, cakes, pasta and related bakery products.
We have found that certain high fiber-containing by-products from the grain milling and processing industries can function as extenders in producing urea-formaldehyde plywood glues or adhesives that are comparable to those made with wheat flours. These materials include cereal grain milling by-products, such as corn bran, sorghum bran, oat hulls, barley hulls, rice hulls, rice bran and wheat bran, oilseed processing by-products such as soybean hulls and cottonseed hulls, and cereal grain processing by-products, such as spent brewers grains, expelled or extracted corn germ and malt husks.
This was an unexpected finding since by-products such as corn bran, oat hulls, soybean hulls, etc., have heretofore been thought to have no industrial utility, and have previously been employed mainly as inexpensive ingredients for animal feeds. These products are generally quite crude in composition and contain portions of the endosperm of the grain as well as high levels of cellulose and hemicellulose. The latter components in particular have not been considered as useful ingredients in adhesive applications.
A novel feature of this invention is that these high fiber products contain less than 50% starch and usually contain less than 30% starch while most amylaceous materials used in plywood adhesives, such as cereal flours and starches, contain from 80 to 100% starch.
An advantage of this invention is that the high fiber products are less expensive than most cereal flours and starches normally used in the manufacture of plywood glues. This feature provides an economic advantage to those associated with the production of plywood glues and the manufacture of plywood.
We have found that the high fiber products of this invention produce urea-formaldehyde plywood adhesives that are comparable in viscosity to those produced with wheat flours. This discovery appears to be related to the unique water absorption properties of the high fiber materials, which properties are two to four times greater than those of cereal flours and starches. This water absorption property provides increased vicosity to the urea-formaldehyde glue system, similar in effect to the properties of the gluten protein which is associated with wheat flours commonly used in this application.
We have found that this unique water absorption property appears to be associated with the composition of the products. In particular, water absorption is related to a combination of the % of pentosans and % of protein of the fibrous products.
We have also found that in some cases the water absorption properties of individual fibrous products are too great, so that the final urea-formaldehyde glue viscosity is too high for practical use. This may appear to be a disadvantage in using these products. However, with the improved method of the present invention, by combining selected amounts of selected fibrous products that produce high viscosity glues with selected amounts of other products such as selected cereal flours or starches, that produce low viscosity glues, which normally are not satisfactory, mixtures that produce satisfactory glue viscosities for urea-formaldehyde plywood manufacture are obtained. We have further discovered that certain selected high fibrous material can be combined to produce mixtures having the proper water absorption properties for use in urea-formaldehyde plywood adhesives.
We have found that an unexpectedly high percentage of a fibrous material yielding high glue viscosities can be combined with those products which produce low viscosity glues to provide a mixture imparting a satisfactory viscosity to urea-formaldehyde adhesives. This negative synergistic action affords the economic advantage of allowing the use of large percentages of the less expensive fibrous materials to produce plywood glues having properties comparable to those glues formulated with the more expensive wheat flours.
The combination of high fiber-containing products with cereal flours or starches offers the additional advantage of utilizing materials in an application where previously none of these products could be satisfactorily employed.